udevd: fix a reversed conditional on global property set

# udevadm control --property=HELLO=WORLD
  Received udev control message (ENV), unsetting 'HELLO'
  # udevadm control --property=HELLO=
  Received udev control message (ENV), setting 'HELLO='

Oh no, it's busted. Let's try removing this one little negation real quick
to see if it helps...

  # udevadm control --property=HELLO=WORLD
  Received udev control message (ENV), setting 'HELLO=WORLD'
  # udevadm control --property=HELLO=
  Received udev control message (ENV), unsetting 'HELLO'

Feels much better now.

src/udev/udevd.c

@@ -1052,7 +1052,7 @@ static int on_ctrl_msg(struct udev_ctrl *uctrl, enum udev_ctrl_msg_type type, co
                 }
 
                 eq++;
-                if (!isempty(eq)) {
+                if (isempty(eq)) {
                         log_debug("Received udev control message (ENV), unsetting '%s'", key);
 
                         r = hashmap_put(manager->properties, key, NULL);